Fat adaptation for exercise – how far can it go?

Who, what & why?

Despite a great deal of discussion about, and anecdotal evidence to support the benefits of a low carbohydrate high fat (LCHF) diet, there have been few studies examining the degree to which athletes’ metabolisms have been altered to take advantage of this dramatic change in fuel for exercise.

A team of researchers, principally from the University of Connecticut, led by Jeff Volek, decided to determine the extent of adaptations towards burning fat as a fuel in elite ultra marathon runners and ironman distance triathletes. Ultra events are the most difficult to fuel solely from carbohydrates, as we carry limited stores in the muscles and liver and can only take in ~300 kCal per hour during exercise. As a group of athletes, therefore, this ultra endurance group could be expected to benefit the most from the adaptation towards burning fats as fuel.

What did they do?

They recruited 20 elite marathoners and ironman triathletes 10 of whom habitually consumed a high carb diet (59% of calories) and 10 of whom had been practicing a low carb (10% of calories) for an average of a year and a half. This period of time is significant because it is generally considered that adapting to a high fat diet takes many months. The athletes were carefully matched for age and performance (which must have taken some doing considering they were elite performers!).

The athlete subjects had their body composition analysed, and resting measures were carried out, including muscle biopsies, blood and metabolic measures.

The athletes were then given a nutritious smoothie with either a high or low carb content depending on which diet they were used to.

A 3 hour treadmill run at 65% VO2 max with two blood and exhaled gas measurement points was followed by further measures during a seated 2-hour recovery period.

What did they find?

Not unexpectedly, the rate of fat use was very significantly higher in the athletes used to a low carb high fat diet before, during and after the run:

Conversely, the rate of carbohydrate use was much lower in the low carb group:

What’s interesting to note is that although the rates of fat and carb burning remained constant in the low carb group, the rate of fat burning increased to complement the decline in carb availability in the high carb group. It was still only 2/3rds of the rate of fat burn by the end of the 3 hours though.

What does it mean?

The most interesting thing about this study is that athletes who had been on a low carb high fat diet for an average of 18 months showed a dramatically increased ability to use fat as fuel during long distance events. Even without considering any other benefits, this is significant because one of the biggest challenges endurance athletes face is simply eating enough to avoid the dreaded wall or bonk.

The rate of fat burning was equivalent to 660 kCal out of a total 745 kCal per hour i.e. a whopping 89% in the low carb group, compared to only 58% peak in the high carb group.

These are really interesting statistics, especially considering that if fully fat adapted, an athlete might need less than 100 kCal per hour during an ultra event, an amount that can easily be ingested without risking any stomach upsets. For habitual high carb diet athletes, the average is closer to 360 kCal per hour, which requires a disciplined ingestion of maximal quantities of glucose + fructose mix throughout the event.

Another interesting finding is that contrary to what you might expect, concentrations of muscle glycogen were the same between the two groups after the 3 hour run. Reasons for this are still speculative, but there are pathways in the body where glycogen can be made from both lactate and glycerol (a primary component of circulating fats).

Not surprisingly, the low carb athletes had 3x higher levels of ketones circulating, showing the extent of low carb adaptation, which is considered to be an evolved response to starvation, allowing mammals to survive (and thrive) on diets low in carbohydrates.

Now if only chocolate digestives were an integral part of a low carb diet…

You may also be interested in this recent guest blog by an expert nutritionist looking at different individuals suitability to a LCHF diet and taking it a step further to reach ketosis.

5 Comments

Hi Leo,
Long term implications for the individuals are I think unknown at this point – LCHF advocates would argue that they will remain healthier and sustain their athletic and mental performance longer into old age. Conventional healthcare wisdom is that high fat diets increase cholesterol leading to a higher incidence of heart disease.
What I personally expect is that some people will really benefit from high fat diets, some will not (for genetic reasons) and that on the whole, western diets will gradually reduce amounts of sugar & refined carbohydrates, and that people will feel better about butter!

Thanks for the post. Ive found my experience on low carb to show the same as the study on longer training rides and races ( no refueling for 3/4 hour races with no drop off in performance). Although for shorter distances under 2 hours my performance has been dramatically decreased. I now follow a low carb diet for most of the day and then eat carbs in the evening with much better results.

Thanks for the interesting post.
What would be interesting is to see what kind of training each of the groups did.
It is my guess the LCHF group probably train at MAF or below. This would also train the body to burn fat for fuel more efficiently.
It would be interesting to see an athlete with HC diet that ate clean (i.e. no highly processed carbs and sugar) and trained at MAF would they yield similar results in fat burning, as they might also be better fat-adapted.

Hi Richie
Thanks for taking the time to read & reply here. You would think that the LCHF group might train at MAF or below, but since they now have significant fat burning at higher intensities than HC athletes, I wonder if that is still as important once they are well fat adapted? Perhaps their body’s just see fat as the fuel of choice even in the presence of carbs (study idea?). I will email the study author to see if they want to reply to your points.